The present invention generally relates to the measurement of induction, and more particularly to inductive vehicle detectors and their applications.
Metal detectors are widely used to locate metallic objects that are buried or otherwise hidden from view in military, forensic, geological prospecting, archaeological exploration, and recreational treasure-hunting applications. They have many industrial uses including proximity and position sensing, and the automated inspection of manufacturing, assembly, and shipping processes. They are the active component in pedestrian screening devices used at airports and other high-security areas to detect the presence of concealed weapons. Inductive vehicle detectors are widely deployed on highways and at intersections for traffic-flow monitoring and control, and at parking facilities for revenue control and access control.
The measurable inductance of a wire-loop is directly proportional to the magnetic permeability of the space surrounding the loop. Non-metallic matter typically has no measurable effect on the magnetic permeability of the space it occupies, while metallic matter can measurably increase or decrease the magnetic permeability of the space it occupies depending on its composition. It is well known in the prior-art to measure the inductance of a wire-loop to detect the presence or absence of metal near the loop. The presence of iron tends to increase the inductance of a wire-loop, while the presence of non-ferrous metal tends to decrease the inductance of a wire-loop.
The variation of inductance typically observed in vehicle detectors of the prior-art is on the order of two-percent of the nominal inductance of the wire-loop, while the electromagnetic noise and thermal drift affecting the wire-loop is of approximately the same order of magnitude. Major identifiable sources of electromagnetic noise include electrical power lines, computing and communications equipment, automotive ignition systems, and cross-talk between wire-loops when two or more sensors are deployed in close proximity to one another.
Prior-art wire-loops are deployed in a plane which is roughly parallel to the surface of the roadway into which they are embedded, and the wire-loops are positioned and shaped so that the variation in the inductance of the wire-loop caused by over-passing vehicles is maximized, while uncertainties due to electromagnetic noise is minimized. Prior-art wire-loops are typically deployed with a rectangular geometry comprising four wire legs. The magnetic field generated by a current flowing in a wire is described by the Biot-Savart law of physics, and is known to form roughly a cylindrical magnetic field around each leg of a wire-loop with a field intensity which diminishes linearly with increasing radial distance from the wire. The two cylindrical magnetic fields produced by opposing legs of a wire-loop tend to cancel each other out with the effect being that the farther the two legs are separated in space, the stronger the composite magnetic field will be above the wire-loop where vehicles are to be detected. However, the vulnerability of the wire-loop to electromagnetic noise also increases as the legs of the wire-loop are separated from each other which results in a generally poor signal-to-noise ratio. Prior-art detectors which are able to reliably detect passenger cars are unable to reliably detect motorcycles, snowplows, large trucks, and other vehicles with high ground clearance because of the uncertainty imposed by ambient electromagnetic noise and temperature drift. In addition to reducing traffic-flow efficiency, this can lead to property damage and personal injury caused by automated parking gates which prematurely close on vehicles having high ground clearance.
The techniques of the prior-art which tend to maximize the signal-to-noise ratio of wire-loops by widely separating the four legs of the loops, and deploying the loop so that the vehicles are detected by all four legs of the loop simultaneously, also tend to destroy the potential of the wire-loops for providing repeatable inductive signatures of detected vehicles because the vehicles are not constrained to pass over the wire-loop in the same way every time. Vehicles passing over prior-art wire-loops at different angles and different lateral offsets to the wire-loop necessarily produce inductive signatures which are different. In order to use an inductive signature of a vehicle to classify or identify the vehicle, it is desirable to constrain the vehicle to produce an inductive signature which is as nearly repeatable as possible. In addition, because it is desirable for vehicles to eclipse the magnetic fields of all four legs of prior-art wire-loops simultaneously, these wire-loops are often designed to be relatively narrow and therefore forfeit the strong signals produced by wheel rims.
In the prior-art, the inductance of a wire-loop is measured by making the wire-loop part of a free-running oscillator circuit which has a frequency determined by the inductance and resistance of the wire-loop. A frequency-counter then counts the number of charge-discharge cycles of the oscillator over a pre-determined period of time. This count is partially a function of the varying inductance of the wire-loop, but also varies with electromagnetic noise and thermal drift. A temperature change in the wire-loop of only 6-degrees Centigrade would typically cause a baseline drift equal to the full-scale of the inductance variations being measured because the resistance of the wire in the wire-loop is temperature dependent.
U.S. Pat. No. 5,523,753 cancels some of the low-frequency components of the electromagnetic noise which are predictably generated by power-lines and which have a basically periodic nature. Low-frequency noise is amplified, high-frequency noise is unaffected, and only 60 inductance measurements per second may be made using this technique. The xe2x80x9ctime-aperturexe2x80x9d of the detector is open for an entire 16.7 ms of each sample which is undesirable for making precision measurements of rapidly varying inductance; the xe2x80x9ctime-aperturexe2x80x9d of the detector is the time during which a change in the inductance being measured will cause a change in the inductance measurement.
U.S. Patent No. 5,491,475 describes the use of magneto-resistive sensors having the capability of distinguishing different magnetic signatures of basic vehicle types. The disclosed magnetometers do not constrain over-passing vehicles to present repeatable signatures which renders them useless for precise vehicle classification and identification applications, and the sensors are shown to be sensitive to vehicles in adjacent traffic lanes which introduces an added element of uncertainty into any signatures recorded. Magnetometers may be used in combination with wire-loops of the present invention where appropriate.
The length of prior-art wire-loops is limited in practice because the loops typically enclose a quantity of pavement material which would tend to destroy larger wire-loops over time due to thermal expansion.
The present invention may be substituted for prior-art metal detectors in any of the previously known applications, and new applications for metal detectors are made possible by the increased speed, precision, and repeatability of inductance measurements characteristic of the present invention. In particular, a wide variety of intelligent traffic-flow monitoring and control applications are now feasible.
Identifying or classifying vehicles by inductive signature is useful in many applications including parking-lot revenue control, screening traffic-flow for potential car-bombs, passive security of restricted communities, and traffic-flow monitoring and control in general.
The capability of making high-precision measurements of the velocity of vehicles traveling on a highway combined with the capability for classification and unique identification of selected vehicles is useful in traffic-law enforcement applications such as the automated screening of traffic-flow for vehicles operating at excessive speeds.
It is a first object of the present invention to provide a wire-loop configuration for vehicle detection which constrains over-passing vehicles to present a substantially repeatable inductive signature.
It is a second object of the present invention to maximize the identifying information contained within an inductive signature.
It is a third object of the present invention to increase the signal-to-noise ratio of induction measurements made of a wire-loop used for vehicle detection.
It is a fourth object of the present invention to measure the inductance of a wire-loop used for vehicle detection in a relatively short period of time with relatively high-precision by a method which is serially repeatable and substantially independent of preceding and succeeding measurements.
It is a fifth object of the present invention to record an inductive signature of an automotive vehicle by making a plurality of successive measurements of the inductance of a wire-loop while the automotive vehicle overpasses the wire-loop.
It is a sixth object of the present invention to provide a configuration of two or more wire-loops for vehicle detection which have an improved capacity to resolve velocity and acceleration profiles of over-passing vehicles.
It is a seventh object of the present invention to use velocity and acceleration profiles of over-passing vehicles to compensate, or normalize, for distortions of the inductive signatures recorded for those vehicles.
It is an eighth object of the present invention to correlate the normalized inductive signature of an unknown vehicle with the normalized inductive signature of a known vehicle to determine if the known vehicle and the unknown vehicle are of the same classification.
It is a ninth object of the present invention to correlate the normalized inductive signature of an unknown vehicle with the normalized inductive signature of a known vehicle to determine if the known vehicle and the unknown vehicle are the same vehicle.
It is a tenth object of the present invention to correlate a sequence of characteristic point magnitudes from an inductive signature of an unknown vehicle with a sequence of characteristic point magnitudes from an inductive signature of a known vehicle to determine if the known vehicle and the unknown vehicle are of the same classification.
It is an eleventh object of the present invention to provide a wire-loop configuration for vehicle detection which substantially overcomes the practical limitations on the length of wire-loops deployed within roadway surfaces.
It is a twelfth object of the present invention to provide a wire-loop configuration for vehicle detection which requires less effort to install and maintain within existing roadway surfaces.
These and other objects of the invention are achieved with an extended blade-type wire-loop configuration for vehicle detection; by cancellation of a large fraction of the electromagnetic noise and thermal drift affecting the wire-loop; and with a high-speed and high-precision method of making a plurality of successive discrete measurements of the inductance of the wire-loop while an automotive vehicle overpasses the wire-loop.